Insulin is a peptide hormone comprised of a two chain heterodimer that is biosynthetically derived from a low potency single chain proinsulin precursor through enzymatic processing. Human insulin is comprised of two peptide chains (an “A chain” (SEQ ID NO: 1) and “B chain” (SEQ ID NO: 2)) bound together by disulfide bonds and having a total of 51 amino acids. The C-terminal region of the B-chain and the two terminal regions of the A-chain associate in a three-dimensional structure to assemble a site for high affinity binding to the insulin receptor.
Insulin demonstrates unparalleled ability to lower glucose in virtually all forms of diabetes. Unfortunately, its pharmacology is not glucose sensitive and as such it is capable of excessive action that can lead to life-threatening hypoglycemia. Inconsistent pharmacology is a hallmark of insulin therapy such that it is extremely difficult to normalize blood glucose without occurrence of hypoglycemia. Furthermore, native insulin is of short duration of action and requires modification to render it suitable for use in control of basal glucose. Established approaches to delay the onset of insulin action include reduction in solubility, and albumin binding.
For example, two commercially available insulin derivatives have been prepared to provide a longer action profile. More particularly, the insulin derivative [GlyA21, ArgB31, ArgB32]insulin was prepared to shift insulin's pI from 5.4 to 6.7 resulting in the peptide being precipitated at physiological pH and thus delaying adsorption and time of action (see Bolli et al., Diabetologia 1999, 42, 1151-1167). However, this insulin derivative has enhanced IGF-1 affinity, leading to increased proliferative actions and the possibility of tumorigenesis. Another commercially available insulin derivative is [LysB29-tetradecanoyl, des(B30)]insulin, wherein LysB29 has been acylated with a C14 fatty acid (Mayer et al., Peptide Science, 88, 5, 687-713). The presence of the fatty acid chain enhances binding of the peptide to serum albumin, resulting in increased plasma half life. However, this derivative suffers the disadvantage of having reduced potency in vivo. In addition, both insulin derivatives exhibit variability in biological action from one patient to the next.
Prodrug chemistry offers the opportunity to precisely control the onset and duration of insulin action after clearance from the site of administration and equilibration in the plasma at a highly defined concentration. The central virtue of such an approach, relative to current long-acting insulin analogs and formulations, is that the insulin reservoir is not the subcutaneous fatty tissue where injection occurs, but rather the blood compartment. This removes the variability in absorption encountered with prior art delayed onset insulin derivatives. It also enables administration of the peptide hormone by routes other than a subcutaneous injection.
Binding of insulin to its receptor will result in biological stimulation, but will also initiate the subsequent deactivation of insulin induced pharmacology through the enzymatic degradation of the insulin peptide. An added advantage of using a prodrug derivative of insulin is that such an approach also extends insulin's biological half life based on a strategy of inhibiting recognition of the prodrug by the corresponding receptor. In spite of these advantages associated with prodrug derivatives, the complex nature of preparing such prodrugs has, until now, prevented the preparation of an efficacious prodrug derivative of insulin. To build a successful prodrug-hormone, an active site structural address is needed that can form the basis for the reversible attachment of a prodrug structural element. The structural address needs to offer two key features; (1) the potential for selective chemical modification and (2) the ability to provide a high degree of activity in the native form upon removal of the prodrug structural element. The insulin prodrugs disclosed herein are chemically converted to structures that can be recognized by the receptor, wherein the speed of this chemical conversion will determine the time of onset and duration of in vivo biological action. The prodrug chemistry disclosed in this application relies upon an intramolecular chemical reaction that is not dependent upon additional chemical additives, or enzymes or enzyme inhibitors.
The ideal prodrug should be soluble in water at physiological conditions (for example, a pH of 7.2 and 37° C.), and it should be stable in the powder form for long term storage. It should also be immunologically silent and exhibit a low activity relative to the parent drug. Typically the prodrug will exhibit no more than 10% of the activity of the parent drug, in one embodiment the prodrug exhibits less than 10%, less than 5%, about 1%, or less than 1% activity relative to the parent drug. Furthermore, the prodrug, when injected in the body, should be quantitatively converted to the active drug within a defined period of time. Applicants are the first to disclose insulin prodrug analogs that meet each of these objectives.